1. Field of the Invention
The invention relates to a D-class power amplifier for use in an audio system.
2. Description of Related Art
In recent years, there is an increasing demand for miniaturization of an amplifier in an audio apparatus. Particularly, power amplifiers tend to have a large casing and a be heavy. This is because in the case of stereo systems called xe2x80x9cmini-compoxe2x80x9d, miniaturization of the product is further required for pursuing a stylish design.
In response to the demand for miniaturization of the power amplifier, a power amplifier using what is called a D-class power amplification has become used popularly.
The D-class power amplification is an amplifying scheme that uses a modulating process such as pulse width modulation (PWM) or pulse density modulation (PDM) performed to a signal supplied to the power amplifier. The signal thus converted into a digital modulation signal, is thereafter amplified and the amplified signal is outputted as an analog signal via a low pass filter. In the case of the D-class power amplifying system, by the on/off-type driving of switching devices at a D-class switching stage before the low pass filter in accordance with the digital modulation signal formed based on the input signal, the signal is amplified so that an electric power efficiency of 100% can be obtained theoretically.
In a D-class power amplifying circuit which operates with positive and negative power supplies, a load current flowing in the low pass filter in the D-class power amplifying circuit and a connected load flows from both of the positive and negative power supplies irrespective of a positive/negative value of an output voltage. In other words, when the positive voltage is applied to the load, the circuit receives an electric power from the positive side power supply and operates so as to supply the electric power to the load and the negative side power supply. In this case, although the flowing direction of the electric power is the consuming direction of the electric power from the power supply to the load for the positive side power supply, it is the direction in which the electric power is regenerated from the load to the power supply for the negative side power supply.
A state where a regenerative current is caused due to the regeneration of the electric power in the D-class power amplifying circuit which operates by both of positive and negative power supplies is shown in FIG. 1.
First, in the diagram, it is assumed that a load current iL flows in a load RL in the direction shown by an arrow in the diagram. Switching devices S1 and S2 at a D-class switching stage in the D-class power amplifying circuit are alternately and repetitively turned on/off at a speed which is higher than a change in load current. That is, it is presumed that a switching frequency at the D-class switching stage is higher than the frequency of the load current.
Since the load current iL flows in an inductor L included in a low pass filter in the D-class power amplifying circuit (a capacitor constructing the low pass filter is omitted in the diagram), even if the on/off states of the switching devices S1 and S2 are switched by inertia of the inductor which obstructs the current change, the flowing direction is not changed in one period of the current iL. That is, as shown in a time chart in FIG. 1, it is possible to consider that the load current iL is time-divided into two currents, i1 flowing in a loop of a positive side power supply +Vcc and i2 flowing in a loop of a negative side power supply xe2x88x92Vcc, in correspondence to the switching of S1 and S2.
Now if we look carefully the directions of the currents i1 and i2, respectively, it will be understood that i1 flows in the direction in which an electric power is consumed from +Vcc, that is, the direction in which the current flows out of the power supply, while i2 flows in the direction in which an electric power is regenerated to xe2x88x92Vcc, that is, the direction in which the current flows into the power supply. A power supply capacitor C2 connected to the negative side power supply xe2x88x92Vcc is, thus, charged and a power voltage rises.
When the load current iL flows in the direction opposite to that in FIG. 1, a similar situation occurs with respect to the loop of the positive side power supply +Vcc, and the power voltage also rises likewise with respect to the positive side power supply.
A ratio of the increase in power voltage due to the regenerative current fluctuates depending on the frequency of the load current, that is, the frequency of the signal which is handled by the D-class power amplifying circuit. Generally, the lower the signal frequency is, the larger the increasing ratio of the power voltage. This is because since the lower the signal frequency is, the less frequently the load current is changed and the longer the time during which the current flows continuously in one direction is, the time during which the regenerative current i2 is integrated to the power supply capacitor C2 in FIG. 1 is also extended.
That is, the build up of power voltage due to the regenerative current becomes a problem, in particular when low frequency components of the signal are handled by the D-class power amplifying circuit. When the power voltage rises largely and exceeds withstanding voltage limits of devices in the D-class power amplifying circuit, then the concern will be the failure of devices in the D-class power amplifying circuit.
The invention is made to solve the drawbacks and it is an object of the invention to provide a D-class power amplifier in which an increase in power voltage due to a regenerative current is suppressed.
According to the invention, there is provided a D-class power amplifier for supplying an amplification signal obtained by amplifying two input signals to two loads which are mutually connected at one end of each load, comprising:
two D-class power amplifying circuits having output terminals connected to the other ends of the two loads, respectively;
a preprocessing circuit for performing a predetermined preprocess to the two input signals and supplying the processed signals to the two D-class power amplifying circuits; and
a power supply circuit for supplying an electric power to the two D-class power amplifying circuits,
wherein one of the two D-class power amplifying circuits executes an anti-phase power amplifying process and the other of the two-D-class power amplifying circuits executes an in-phase power amplifying process,
the preprocessing circuit executes a process for equalizing amplitudes of two input signals in a low frequency band, and
a node of the two loads is connected to a potentially neutral point of an output voltage from the power supply circuit.